1-(7-Chloro-1,4-dihydroquinolin-4-ylidene)thiosemicarbazide and its hydrochloride: evidence for the existence of a stable imine tautomer in the solid state of 4-aminoquinoline free bases, an anomalous case in nitrogen heterocycles

In the solid state, crystals of both 1-(7-chloro-1,4-dihydroquinolin-4-ylidene)-thiosemicarbazide-methanol-water (2/1/1), 2C(10)H(9)ClN(4)S center dot CH3OH center dot H2O, (I), and its hydrochloride salt {systematic name: [(7-chloro-1,4-dihydroquinolin-4-yl-idene)azaniumyl]thiourea chloride}, C10H10ClN4S+center dot Cl-, (II), assume the imine tautomeric form, contrary to other 4-amino-7-chloroquinolines. Of particular interest are the N-C bond lengths, which have appreciable double-bond character, and the C-N-C aromatic ring bond angle. Both of these parameters have been studied extensively in 4-amino-substituted quinolines. The crystal structures of (I) and (II) in this study provide interesting examples of the amino-imino tautomerism which exists in this class of compound and is, to the best of our knowledge, hitherto unreported.